1. Field of the Invention
This invention relates generally to electrochemical reactors for conducting reduction and oxidation reactions in respective positive and negative liquid electrolytes, without gas evolution at the electrodes. More specifically, the invention relates to the operation of a membrane-separated, bipolar multicell electrochemical reactor implementing a redox flow battery system, although it may be useful also for different systems.
2. Description of Related Art
Redox flow battery systems are increasingly attracting interest as efficient energy storage systems. Among redox couple candidates, the all vanadium redox system is one of the most preferred.
Structurally, the electrochemical reactors that have been proposed for redox flow battery systems, have been derived from the electrochemical reactor structures developed for general electrolysis processes, the only adaptation having concerned the materials employed as electrodes.
Generally, the electrochemical reactors used as redox batteries are conventionally composed of a stack of bipolar plate electrode elements separated by ion exchange membranes, defining a positive electrolyte flow chamber on one side of each membrane and a negative electrolyte flow chamber on the opposite side thereof. The stack of bipolar elements is assembled together in a filter-pass arrangement between two end electrode elements.
Commonly, the elements have a frame provided with coordinated through holes forming inlet and outlet manifolds for the two electrolytes that are circulated in a parallel mode through the positive electrolyte flow chambers and the negative electrolyte flow chambers, respectively.
The elements are conventionally mounted and operated in a vertical position.
The parallel flow of the two electrolytes through the respective flow chambers poses serious problems in terms of minimization of so-called stray or by-pass electric currents in uninterrupted liquid veins of electrolyte, due to the fact that the electrolyte present in the manifolds offer innumerable paths for these by-pass or stray currents, driven by mutual voltage differences existing among the various bipolar elements functioning in electrical series between the two end electrodes on which the full battery voltage difference insists. By-pass or stray currents decrement the energy efficiency of the conversion system, but more seriously they cause severe corrosion phenomena on conductive parts (e.g.: carbon) because of abnormally high half-cell voltages at the conductor surface.
On the other hand, the redox system require nonnegligible electrolyte flow rates through the flow chambers of the reactor in order to maintain optimal half-cell reactions conditions at the electrodes and this requirement may imply the necessity of operating the bipolar electrochemical reactor at relatively high positive pressures.
A different architecture, object of the prior patent application PCT/IT99/00195 of the same applicant, contemplates alternately stacking a bipolar electrode holding subassembly and a membrane holding subassembly, laying them horizontally.
The alternate stack of elements is piled over a bottom end element and the stack is terminated by placing over the last membrane holding element a top end electrode element. The two end electrode elements are then compressed over the stack by tightening a plurality of tie rods, conventionally arranged around the perimeter of the stacked elements, according to a common practice in tightening a filter-press stack in a hydraulically sealed manner, by virtue of the gaskets operatively installed between the coupling faces of the frames of the various elements. The battery may be operated with the piled elements laying horizontally.
In the above noted architecture, each bipolar plate electrode holding element and each ion exchange membrane separator holding element includes a substantially similar rectangular frame piece, made of an electrically nonconductive and chemically resistant material, typically of molded plastic material, having on its upper (assembly) face grooves for receiving O-ring type gasket means, and having through holes and recesses in coordinated locations disposed along two opposite sides of the rectangular frame forming, upon completion of the assembling, ducts for the separate circulation of the negative electrolyte and of the positive electrolyte through all the negative electrolyte flow chambers and all positive electrolyte flow chambers, respectively, in cascade.
The negative electrolyte enters along a first side of a negative electrolyte flow chamber, flows through the chamber toward the opposite or second side thereof, exits the chamber, flows through the coordinated holes through the frame holding the electrode and through the frame holding the next membrane separator, reaching the level of the next negative electrolyte flow chamber and enters it from the same second side through which it exited from the previous negative electrolyte flow chamber and exits this next negative electrolyte flow chamber from the same first side it entered the previous negative electrolyte flow chamber, to flow through coordinated holes through the next pair of frames to the level of the next negative electrolyte flow chamber and so forth. The same flow path is arranged also for the positive electrolyte, either in a xe2x80x9ccountercurrentxe2x80x9d or in an xe2x80x9cequicurrentxe2x80x9d mode through the battery.
In practice, the bipolar electrochemical reactor does not have inlet and outlet manifolds for the two electrolytes, on the contrary, the electrolytes flow through the respective flow chambers in a zigzag path, that is essentially in hydraulic series or cascade mode instead than in hydraulic parallel mode.
In this way, by-pass current may only be xe2x80x9cdrivenxe2x80x9d by a voltage difference of about one-cell voltage and it does not cause any corrosion on conductive parts.
Pitting corrosion is not the only consequent of by-pass currents.
By-pass currents lower the overall efficiency of the charging and discharging processes because by-pass currents represent parasitic discharge mechanisms of the flow redox battery.
A typical way of using flow redox battery systems is to accumulate energy by transforming electrical energy into chemical energy during periods of excess electrical power generating capabilities (for example solar energy conversion during daylight hours or excess electrical power capabilities during night time hours in power generation plants) and to deliver accumulated energy in the form of electrical power when required by a load circuit.
Often, in the normal daily cycling of a flow redox battery system there may be prolonged periods of inactivity that is periods when the battery is not charging nor supplying electrical power to an external load circuit. During these idle periods, the pumps that circulate the positive electrolyte and the negative electrolyte through the cell are switched off to save energy and the electrolyte in the battery remain still.
In these conditions, the volumes of electrolytes, contained in the respective compartments of the cells composing the battery stack, supports the by-pass currents that typically are practically entirely confined within the electrolyte battery stack and therefore tends to slowly decrement their state of charging.
As a consequence, if electrical energy is required by the utilizer circuits, the system may take several minutes of xe2x80x9cstart-upxe2x80x9d before becoming ready to provide the appropriate output voltage, a condition that is attained upon a complete refreshing of the electrolytes in the compartments of the battery stack upon resuming their forced circulation by switching on the respective pumps.
This phenomenon may impose the presence of auxiliary battery systems for providing the electrical power necessary to operate the electrolyte pumps at least during the xe2x80x9cstart-upxe2x80x9d period when the output voltage of the battery may have dropped to an insufficient level because of the intervening discharge of the electrolytes volumes retained in the respective compartments during a protracted period of idleness.
Of course, in applications where this is intolerable, a possible solution would be to maintain a trickle charge current through the battery deriving such a maintenance power from an auxiliary source or maintain the electrolyte pumps in function to prevent a deep discharging of the electrolytes in the battery compartments. Both solutions are penalizing in terms of energy requirement, especially in those applications that contemplate prolonged periods of inactivity of the system.
Another critical aspect that has been observed is the ability of exploiting the fullest nominal cell area of the battery.
This criticality manifests itself at relatively high regimes of operation, that is when the level of current flowing through the battery, either in a discharge direction or in a charging direction, approaches the rated maximum value that, apart from other design parameters, is directly tied to the cell area (or active electrode area).
It has been found that the major factor effecting the ability of a battery to support relatively high currents while maintaining an acceptable reversibility is the formation of velocity gradients within the electrolyte flowing in the relatively narrow gap between the bipolar wall and the ion exchange separator in the cell compartments.
The problem becomes even more critical when the active electrodes are in the form of a felt or of alike open structures crossed by the streaming electrolyte forced by the pumps.
Formation of velocity gradients in the body of electrolyte within an electrode compartment implies that numerous zones of the nominal cell area will tend to contain a relatively depleted (that is less charged) electrolyte than other zones where the pumped electrolyte tends to flow preferentially.
In extreme conditions, this phenomenon may in practice reduce the effective cell area (or active electrode area) to a fraction of the nominal size.
At high electric current regimes, the phenomenon manifests itself in a severe drop of the output voltage, during a discharge phase and in an abnormal rise of the voltage across the battery, during a charge phase.
To cure this problem and optimize the pumping xe2x80x9ccostxe2x80x9d, it is a trivial expedient to increase the pumping rate of the electrolyte in function of the current through the battery. However, even this approach implies a remarkable penalty in terms of overall efficiency due to an augmented power absorption by the pumps.
Conventional considerations on hydraulic systems and the objective of limiting the power expenditure for pumping the electrolytes have led designers to minimize the flow rates of the electrolyte to the lowest value compatible with the requisites of providing for an adequate refresh of the electrolyte throughout the area of the cell compartments at the particular current of operation. The flow of electrolytes in known electrolysers and in particular in redox flow batteries is laminar in order to take place with minimum pressure drops.
Vis-à-vis this state of the art, it has now been found that by realizing or installing check valve liquid vein interrupters in each compartment of the battery the above discussed phenomenon of slow discharge of the retained volumes of electrolytes during long periods of inactivity of the battery, with the electrolyte pumps stopped altogether can be practically eliminated with the effect that the battery is perfectly ready to deliver electric power immediately upon request even after prolonged periods of inactivity.
Moreover, by virtue of liquid vein interrupters present on each compartment in either an outlet or an inlet port substantially preventing by-pass current during a not pumping phase, a definitely augmented overall efficiency may be achieved by pumping the electrolytes through the compartments of a battery stack intermittently, in other words in a pulsed manner, with a certain duty-cycle.
According to this embodiment, during normal functioning, relatively brief pumping phases at relatively high flow rate alternated to phases of not pumping. In this way, providing for a volumetrically adequate refreshing of the electrolytes present in the battery compartments, the formation of gradients in the bodies of electrolyte, each representing the volume of electrolyte currently contained in a compartment of the battery, is contrasted.
It has been observed that, a proportionately augmented flow rate of pumping during the pumping phase of each cycle, such to cause a turbulent flow that may last practically for the entire pumping phase of each cycle or develop for just a fraction of the duration of the brief pumping phase, appears to be instrumental in xe2x80x9cdestroyingxe2x80x9d any incipient tendency of the electrolytes streaming through the compartments of the battery of assuming preferred flow patterns.
In case of the existence of xe2x80x9cfree flowxe2x80x9d gaps in the compartments, substantially between the face of the active electrode and the ion exchange membrane cell separator, the slight increase of the pressure drop associated to the switching from a laminar to a turbulent flow is abundantly counterbalanced by the markedly improved conditions of operation and a remarkable improvement of the reversibility characteristic of the battery and an overall net increase of conversion efficiency are observed.
Most surprisingly it has been observed that in case of a cell structure with xe2x80x9cno free flowxe2x80x9d gaps in the compartments, that is when a felt electrode or otherwise porous electrode mass occupies practically the entire space between the bipolar (or end) wall and the ion exchange membrane cell separator, the pressure drop along the battery may even decrease as compared to the case of a conventional continuous pumping under uninterrupted and purely laminar flow conditions.
An explanation of this may be attributed to the peculiar conditions under which the flow of the electrolyte occurs through a porous mass of solid electrode fiber or particulated material, as noted in the volume xe2x80x9cPerry""s Chemical Engineer Handbookxe2x80x9d chapter 5.53: xe2x80x9cFlow through fixed beds of granular solidsxe2x80x9d.
Irrespective of the physical explanation of such a behavior, the beneficial effects of circulating the electrolytes through the cell compartments of the battery in a pulsed manner and preferably with periods of turbulent flow within the compartments, are sensible.
According to a preferred embodiment of the invention, the realization of check valve means in the inlet and/or in the outlet port or ports of each compartment of the cell composing the battery stack, is simple and inexpensive. In a simplest form, these check valves may be realized by confining a ball of noncorrodible material such as Teflon, polyethylene and any other suitable plastic material, provided its density is sufficiently greater than the density of the electrolyte, within a xe2x80x9chousingxe2x80x9d defined upon assembling together the elements of the battery stack which will permit the lifting off of the ball from a valve seat on which it rests by gravity, upon activating the circulation pumps. Of course, electromagnetically or even magnetically operated check valves could also be employed, although sensibly complicating the design.